Energy transfer (EnT)-mediated [2 + 2] photocycloadditions

The [2 + 2] cycloaddition of alkenes is thermally forbidden and traditionally achieved through high-energy UV irradiation. However, recent advances in triplet energy transfer (EnT) enable these cycloadditions under visible light, offering mild, selective and sustainable access to cyclobutane scaffolds. With an emphasis on the key mechanistic aspects, this review highlights recent (2022-2025) advances in energy-transfer mediated inter- and intramolecular [2 + 2] photocycloadditions of olefinic substrates with efficient strategies for the construction of cyclobutane frameworks. The intermolecular reaction section is categorized into alkene-alkene [2 + 2] cycloadditions, alkene-heterocycle [2 + 2] cycloadditions, and aza-/thia-Paternò-Büchi reactions. The intramolecular reaction section is subdivided into [2 + 2] cycloadditions involving hetero-/carbocycles and acyclic analogues. In addition, recent EnT-mediated [2π + 2σ] cycloadditions are discussed, exploring new opportunities to activate strained substrates.